Steel filament members are used as reinforcing materials in various rubber products such as radial car tires, conveyor belts and high-pressure hoses.
This `steel filament member` includes steel wires or steel filaments (hereinafter called steel wires) 0.10 to 0.40 mm in diameter and plated with brass to improve the adhesion of rubber thereto and steel cords of various structures made by twisting together a plurality of these steel wires. Hereinafter, in this invention, when both steel wires and steel cords are being referred to, the term steel filament member will be used.
Conventionally, steel wires have generally been made from a carbon steel wire rod material containing 0.70 to 0.75 wt % carbon, and in relation to the wire diameter d (mm) the tensile strength Y (N/mm.sup.2) has been Y.gtoreq.-1960d+3290, as shown in FIG. 1.
However, recently, lightening of rubber products has been being demanded. For example in the case of car tires, lightening is being demanded for the sake of fuel economy. As a way of achieving this, increasing the strength of steel cords and steel wires constituting them used in large amounts as a reinforcing material in rubber products and reducing the amount used per unit is effective. For example, a high tensile steel wire of diameter 0.25 mm and tensile strength over 3090N/mm.sup.2 or diameter 0.35 mm and tensile strength over 2890N/mm.sup.2 is suitable.
Conventionally, such high tensile steel wires have generally been made by drawing a high carbon steel wire rod material containing over 0.80 wt % carbon. For example, in the case of a carbon steel wire rod containing 0.82 wt % carbon, a steel wire of the above-mentioned strength has been made by drawing the rod to a total area percentage reduction of about 96.0 to 97.0%.
The reason why a high carbon steel has been used is that when a carbon steel wire rod material whose carbon content is 0.70 to 0.75% is used, because the strength of the starting wire rod material itself is low, the strength of the wire after final heat treating is carried out also naturally is lower than that of one having a high carbon content. Therefore, to obtain a steel wire of the same strength as a steel wire manufactured with a high carbon steel wire rod material it is necessary to adopt a particularly high area percentage reduction (degree of working) in the drawing process.
However, when such a high degree of working is adopted, although the strength of the steel wire increases as a result of work hardening, on the other hand, defects arise inside the steel wire and consequently sometimes its toughness deteriorates and its tensile strength falls. As a result, problems arise such as that breakage occurs during the drawing process and it is not possible to carry out a predetermined working or that, even if a steel wire of the target strength is obtained, breakage occurs frequently in the step of twisting together a plurality of filaments of the steel wire to make a steel cord and consequently it is not possible to make the steel cord.
Also, when the steel wire is used as a reinforcing material for a high-pressure hose, there has been the problem that the steel wire breaks and trouble consequently arises during a forming process for giving the steel wire a spiral form or during knitting of the steel wire into a braid in the process of molding the hose.
For reasons such as this it has not been possible to manufacture the kind of high tensile steel wire shown in FIG. 1 obtained with a high carbon steel wire rod material containing over 0.80 wt % carbon when using a carbon steel wire rod material whose carbon content is 0.70 to 0.75 wt %.
Accordingly, high carbon steel wire rod materials of carbon content over 0.80 wt % have been used to obtain the required strength characteristics, but using a high carbon steel wire rod material whose carbon content is over 0.80 wt % inevitably increases raw material costs. Furthermore, there has been the problem that the higher the carbon content is the more segregation tends to occur during the steel-making process, and also in the process of manufacturing a steel wire by working the obtained starting wire rod material the heat treatment becomes difficult and the adverse influence of non-metallic inclusions in subsequent drawing and twisting steps is large.
Explaining this further, the above-mentioned `toughness` of the steel wire is extremely important. The reason for this is that when deterioration in toughness exceeds a certain limit, as well as the processing-related problems mentioned above occurring, the fatigue resistance also falls sharply. Furthermore, the problem of toughness of a steel wire is also related to the demand for cost reduction. That is, as a cost-reducing measure, it is effective to reduce the number of steel wires constituting a steel cord and adopt a simple structure such as for example a 1.times.3 structure. However, when the number of steel wires is reduced, to maintain a required strength of the steel cord it is necessary to increase the diameter of the steel wires in addition to raising the strength of the steel wires. But when the diameter of a steel wire is large, due to the filament diameter effect its fatigue resistance with respect to bending falls greatly. To solve this problem it is essential that the steel wire have good toughness.
However, hitherto, there has not been a measure by which it is possible to correctly determine the toughness of a manufactured steel wire. That is, the toughness of a steel wire has generally been evaluated by the number of turns (the twist value) until the steel wire breaks when it is twisted in one direction. However, in practice, even when steel wires having substantially the same twist value are used there has often been dispersion, such as in whether problems like breakage occur or do not occur in the process of twisting together steel wires to make a steel cord. Thus, determinations of toughness limits have lacked reliability. However, correct determination of toughness using other characteristics instead of this as parameters, namely elongation and contraction values on breaking, has been problematic.